The present invention is related to a vehicle direction correcting apparatus for correcting a measurement error of a vehicle direction detected by a relative direction detecting means such as an angular velocity sensor used in a navigation system for a vehicle, namely for correcting the measurement error in such a manner that a drift error is eliminated.
Conventionally, as the angular velocity sensor used in such a vehicle navigation system, such an angular velocity sensor is employed to detect a change in vehicle direction, and a position of the own vehicle is displayed on a map under representation based on this direction change detection result, so that a driver of this vehicle may recognize drive (travel) direction of the own vehicle.
In such a case that the vehicle direction is not changed, for instance, while the vehicle is stopped, or driven along a straight path, an output voltage of an angular velocity sensor must become zero. However, a drift error happens to occur due to adverse influences caused by temperatures and humidity. As a result, the output voltage of this sensor cannot become completely zero, because of this drift error. That is, the central (neutral) point potential which is a reference voltage of the angular velocity sensor could not become completely zero.
To avoid this problem, the vehicle direction correcting apparatus has been proposed by which the sensor output voltage of the angular velocity sensor is corrected so as to eliminate the drift error contained in this sensor output voltage.
In this vehicle direction correcting apparatus, a detection is made of such a condition that there is no change in the vehicle direction, for instance, while the vehicle is stopped, or driven along the straight path. Then, the output voltage of the angular velocity sensor obtained under this condition is set to the central point potential. In other words, since the central point potential of this angular velocity sensor is not set to a constant, but is variable, this conventional vehicle direction correcting apparatus can correct the sensor output voltage in such a manner that the drift error contained in the sensor output voltage of this angular velocity sensor is eliminated.
Therefore, in such a vehicle direction correcting apparatus, since it is required to detect such a condition that the vehicle is stopped, or driven along the straight path, various detecting methods have been employed with use of the GPS system, the CD-ROM, and the vehicle speed sensor employed in the navigation system.
In case of that the GPS system of this navigation system is used, it is possible to detect such a condition that the vehicle is stopped, or driven along the straight path based upon the direction information acquired from the GPS satellite.
Also, in case of that the CD-ROM of the navigation system is used, when the road on which the vehicle is presently driven can be recognized as the straight road on the map by performing the map matching process based on the map data saved on this CD-ROM, it is possible to detect such a condition that the vehicle is driven along the straight road.
Also, in case of that the vehicle speed sensor of the navigation system is used, the occurrence frequency of the vehicle speed pulse corresponding to the speed change amount of the vehicle derived from this vehicle speed sensor is monitored. Then, it is possible to firmly detect such a condition that the vehicle is stopped based upon this occurrence frequency.
Next, the vehicle direction correcting apparatus with using the vehicle speed sensor will now be explained. FIG. 5 is a timing chart for showing the operations of the conventional vehicle direction correcting apparatus for producing the central point potential of the angular velocity sensor by using the vehicle speed sensor.
In such a conventional vehicle direction correcting apparatus, as indicated in FIG. 5, the stop detecting time period "X" defined by a constant time duration in the vehicle speed sensor so as to monitor the occurrence frequency of the vehicle speed pulse. When stopping of the occurrence of the vehicle speed pulse is detected during this stop detecting time period X, the output voltage of the angular velocity sensor outputted when the occurrence of the vehicle speed pulse is stopped during the correction time period "Z" after this stop detecting time period X is set to the central point potential.
As previously explained, according to the conventional vehicle direction correcting apparatus, for instance, such a condition that there is no change in the vehicle direction, e.g., while the vehicle is stopped, or driven along the straight path is detected. At this detection time, the output of the angular velocity sensor is set as the central point potential. In other words, since the central point potential of this angular velocity sensor is not made constant, but is variable, the output voltage of the angular velocity sensor can be corrected based on this central point potential in such a manner that the drift error contained in this sensor output voltage is eliminated.
However, in accordance with the above-explained conventional vehicle direction correcting apparatus, such a condition that the vehicle is stopped, or driven along the straight path is detected, so that the output of the angular velocity sensor is set as the central point potential. Then, the output voltage of the angular velocity sensor is corrected based on this central point potential in such a manner that the drift error contained in this sensor output voltage is eliminated. When such a condition that the vehicle is stopped, or traveled along the straight road is detected based upon the GPS direction information, the precision of the direction information is deteriorated because of the following reasons. For instance, since the vehicle is driven through the city, or the long-time drive stop condition more than 4 hours is continued, the satellite signal unreceivable condition is continued between the GPS satellites and the vehicle, so that no direction information could be acquired. As a result, there is such a first problem that the sensor output voltage could not be corrected in such a manner that the drift error of the angular velocity sensor is eliminated.
Also, in a case of that the vehicle is stopped is detected by executing the map matching process operation by the CD-ROM, since the precision of the map data recorded on this CD-ROM is limited, when the vehicle is traveled on a road which is not mapped on the map data, the travel direction of the vehicle cannot be recognized. As a result, there is such a second problem that the sensor output voltage could not be corrected in such a manner that the drift error of the angular velocity sensor is eliminated.
Further, in a case of that the vehicle being stopped is detected based upon the vehicle speed pulse derived from the vehicle speed sensor, the time period during which the vehicle is stopped is prolonged to some extent in order to accurate the detection while the vehicle is stopped. When such a sufficient detection time period cannot be secured, there is such a third problem that the sensor output voltage could not be corrected in such a manner that the drift error of the angular velocity sensor is eliminated.
In other words, according to such a conventional vehicle direction correcting apparatus, the output voltage of the angular velocity sensor derived while the vehicle is stopped, or traveled along the straight path is set as the central point potential. When the drift error contained in the output voltage derived from the angular velocity sensor is corrected based upon this central point potential, there is a fourth problem that various electric appliances belonging to the navigation system other than the angular velocity sensor, for instance, GPS, CD-ROM, and vehicle speed sensors should be utilized.
Also, according to the above-described conventional vehicle direction correcting apparatus, for example, when the vehicle is parked on a vehicle direction changing apparatus installed in a parking lot, such a special condition may occur. That is, the direction of this vehicle would be changed although the vehicle speed is 0 and the parking break is actuated. In this case, there is a fifth problem that the drift error contained in the output voltage derived from the angular velocity sensor is further increased.